I suppose it would be Core... it currently uses almost zero images(aside from the logo) and technically all of our extra JS is completely optional. You can disable things like the floating header, collapsing nodes and it'll be just like the default style to an extent(loading wise).
Core uses a custom color palette palette(a single palette) to control the orange across the style, and a css gradient for the darker gradient such as sidebar headings/nav bg ect.
That being said we haven't gone in and removed every image gradient on XenBase like we did Flat Awesome. A lot of the gradients were removed in some of the standard properties but the ones where they were hard coded for oddball pages we didn't remove those.